This is the 2007 version. Click here for the 2017 chapter 03 table of contents.

Temperature Variations and Sleep

Are sleep rhythms triggered by the environment? What happens to sleep rhythms if people are cut off from all day/night cues? To find out, Czeisler, Weitzman, Moore, Zimmerman, and Knauer (1980) studied 12 male subjects who lived from 16 to 189 days in rooms deep below the earth, cut off from any indication of time. The subjects set their own daily rhythm. They turned lights on and off, got up and went to sleep when they pleased, without any time clues. All 12 deviated from the usual 24 hour rhythm, but all settled on a daily rhythm. Most ended up with a day that was about 25 hours long.

What did Czeisler and colleagues find out by depriving male subjects of time clues?

The subjects showed regular temperature variations during the day. Their body temperatures rose to a peak once a day and fell to a low point once a day. Almost all the subjects woke up when their body temperatures were rising.

The length of time each subject slept did not depend on how long the subject stayed awake before sleeping; instead, it depended on body temperature when he fell asleep. If a subject went to sleep during his low-temperature point of the day, he would sleep an average of 7.8 hours. He woke up with his next temperature rise. If his body temperature was high when he went to sleep, he slept through the hot temperature period then through the next low temperature period, and again woke up when his body temperature was rising. Men who did that slept an average of 14 hours.

Why might it be wise to go to bed when your temperature is low?

This pattern suggested that long sleepers-people who average 10 or more hours of sleep in a 24 hour period-might be people who go to sleep when their temperature is high. If you want to feel refreshed after only six to eight hours sleep, it is best to go to sleep when your body temperature is low. This is the time when most people feel tired and unable to concentrate on work, anyway. It is also when they get the most REM sleep in the shortest time.

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